/**
 * Calls a defined callback function on each combination of elements from
 * two arrays to find matching pairs, and returns a new array containing
 * both unmatched values from this array and matched values.
 *
 * @param other - The other array to match (join) against.
 *
 * @param on - A function that accepts two arguments - the pair of elements
 *  from the two arrays. The function should return `true` if the
 *  pair of elements match (should join). The leftJoin method calls the
 *  function one time for each combination of elements from the two arrays.
 *
 * @returns An array of left and right pairs.
 */
export declare const leftJoin: <T, U>(other: readonly U[], on: (left: T, right: U) => boolean) => (arr: readonly T[]) => ({
    left: T;
    right: U;
} | {
    left: T;
    right: null;
})[];